@mixin respond-to($type, $m: max-width) {
  @if $type == handheld {
    @media screen and (max-width: 767px) {
      @content;
    }
  } @else if $type == webkit {
    @media screen and (-webkit-min-device-pixel-ratio: 0) {
      @content;
    }
  } @else if $type == nonhandheld {
    @media screen and (min-width: 769px) {
      @content;
    }
  } @else {
    @media screen and ($m: $type) {
      @content;
    }
  }
}

@mixin retina-display() {
  @media screen and (-webkit-min-device-pixel-ratio : 1.5), screen and (min-device-pixel-ratio : 1.5) {
    @content;
  }
}